Decimal to Double conversion in Swift 3

NSDecimalNumber and Decimal are bridged

The Swift overlay to the Foundation framework provides the Decimal
structure, which bridges to the NSDecimalNumber class. The Decimal
value type offers the same functionality as the NSDecimalNumber
reference type, and the two can be used interchangeably in Swift code
that interacts with Objective-C APIs. This behavior is similar to how
Swift bridges standard string, numeric, and collection types to their
corresponding Foundation classes. Apple Docs

but as with some other bridged types certain elements are missing.

To regain the functionality you could write an extension:

extension Decimal {
    var doubleValue:Double {
        return NSDecimalNumber(decimal:self).doubleValue
    }
}

// implementation
let d = Decimal(floatLiteral: 10.65)
d.doubleValue